miniflare: old Pool not closed when runtime entry URL changes

[petebacondarwin]

When previousEntryURL differs from the new #runtimeEntryURL at packages/miniflare/src/index.ts:2143-2150, a new Pool is assigned to this.#runtimeDispatcher without first closing/destroying the previous Pool instance. This is a minor resource leak — the old Pool's connections will eventually time out, but it would be cleaner to explicitly close it. The dispose() method at line 2791 also doesn't explicitly close the dispatcher, though workerd shutting down implicitly terminates the connections.